This invention relates to numerically controlled machine tools for effecting relative movement between a work piece carrying plate and a tool carried by a head, so that the tool can engage the work piece at predetermined intervals along a pattern stored in digital form in a memory.
The term "machine tool" must be interpreted in a wide sense. In fact, the invention is of particular utility when used for industrial sewing, pricking or stitching.
Pricking machines are used for decoration of material in sheet form, for instance for decorating the uppers of shoes by pricking holes along definite patterns, identical for all the shoes of a same series.
To automate this operation, pricking machines with digital control and a memory have already been constructed which prick holes in the shoes following a line which was first of all determined by a stylist, then given a material form on a drawing which is finally used for preparing the control program for the machine.
A prior art system of that type comprises an industrial sewing machine and a general purpose computer such as a PDP 8. The use of such a computer considerably increases the price of the equipment. The program is prepared by placing the drawing on a movable table above an optical viewfinder. An operator controls the movements of the table, typically through a micro joystick, so as to make the drawing pass under the viewfinder and he periodically memorizes points between which the computer will later interpolate for reconstituting the drawing. If the latter is complex, loading the information into the memory requires more than an hour for a shoe.
Another prior art machine uses a punched tape as a memory. The preparation of this tape from a template requires a computer which is not available to most users; the drawing should be sent to the machine manufacturer for preparation of the tape.
It is an object of the invention to provide a machine tool which does not require a computer for loading the data into the memory or for restitution; it is another object to provide a machine which allows a simple rapid and accurate storage of the pattern in digital form.
For that purpose, there is provided a machine tool for storing a digital record of a pattern and effecting a mechanical operation on work pieces according to the stored pattern, which comprises a work piece support table; a head carrying a tool for carrying out said mechanical operation; first and second drive means for effecting relative movement of said table and head along a first and a second coordinate axes, respectively; first and second transducer means for providing electrical signals representative of the amount of movement of said first and second drive means, respectively; an optical follower carried by said head in an invariable position with respect to said tool, said follower being constructed for receiving the image of said pattern as represented by a linear drawing and for controlling said first and second drive means for said follower to follow said pattern; digital memory means; analog-digital conversion means for conversion of analog electrical signals into or from digital signals storable in said memory means; and switch means having a first and a second condition, which in one condition route the signals from said transducer means to said conversion means and memory means for digital storage of the pattern followed by said optical follower and, in the other condition, cause control of the motor by the signals stored in digital from in the memory means through said transducer means and conversion means, whereby the linearity defects of said transducer means and conversion means are compensated.
It will be appreciated that the possible defects of linearity of the transducer means and the conversion means are automatically corrected, since all of them come into play for loading the memory for the restitution, on the sole condition that the work piece is placed on the table so that the linear pattern to be followed is in the same position as in the drawing. This constraint is in fact not troublesome at all, any position suitable for the work piece being also adapted for the drawing.
Once the loading operation is begun, it continues automatically, the optical follower travelling over the whole of the line traced on the drawing. Once the pattern has been memorized, it will be used for a great number of repeats.
The transducer means must have as high a resolution as possible. For this reason, plastic or ceramic track potentiometers will in general be used rather than wound potentiometers. On the other hand, a high level of linearity of the A/D conversion means is not required. The number of quantization levels and binary digits representing the amount of movement will be selected depending on the accuracy desired.
The invention will be better understood from the following description of a preferred embodiment thereof.